Deep overhangs and standing-seam roofing are among the features of the homes at Cobb Hill Cohousing that will help to ensure a long life. Photo: Alex Wilson. Click on image to enlarge.
Number 6 in my list of the top-10 green building priorities is...
Use construction details that control moisture
Careful design and construction is the top priority in creating a more durable home--and often the most important issue is how we manage moisture. This is a big part of the focus of "building science." The building enclosure (walls, foundation, roof) has to be designed to a) keep moisture out, and b) allow any moisture that does get in to dry out. As we have made our homes tighter and better-insulated over the past several decades, this has become even more important. (The leaky, unheated homes our grandparents built could easily dry out because air readily flowed through the walls.)
Building science is a complex field that is evolving quickly as we learn more about moisture and air movement through buildings and building assemblies--far beyond the scope of this column. But here are some examples that will help to illustrate the concept:
Provide deep roof overhangs to keep moisture away from the walls and foundation.
Provide good drainage around the foundation, and slope the ground away from the house.
Always provide a "drainage plain" or "rain screen" when designing and building walls. This air space between the siding and sheathing allows siding to dry out between rain events and prevents water vapor from being driven into the wall cavity from the exterior.
Properly flash around windows and other wall and roof penetrations. Specialized flashing products are available to make this process a lot easier than it used to be.
Provide an "air barrier" in the building enclosure that blocks air flow. Experts used to suggest a "vapor barrier," but blocking airflow is more important than stopping vapor diffusion. An air barrier can still be vapor-permeable, allowing moisture to escape over time.
Avoid moisture sources in the home (for example, provide quiet bathroom fans that will be used while showering, install an outside-venting range hood fan, and in humid climates insulate even cold water pipes to prevent condensation).
Select durable products and materials
Along with design and construction, the products and materials we install in a home can influence durability. We focus a lot of attention on selecting green building materials (see my #8 priority). When a product has high recycled content, for example, it not only reduces the energy and environmental impacts of extracting the raw materials that would otherwise be required, but it also helps keep material out of the waste stream. In my opinion, though, it's an even higher priority to use very durable materials.
If material A will last three times as long as material B, we have three times as long to amortize the environmental impacts that were involved in producing that material. So even if material A took twice the energy to produce, our selection of that material will have a net benefit over the long term.
Fiber-cement siding, for example, costs a lot more than vinyl siding, but it should last a lot longer. The same goes with high-quality, standing-seam metal roofing or slate shingles, compared with asphalt shingles. There is usually a higher up-front cost for more durable materials, but that extra cost is repaid over the long term--both monetarily and environmentally.
Reuse existing buildings
Part of ensuring durability is renovating older buildings rather than building new. Dramatically reducing the energy consumption of existing buildings makes them relevant to future resource constraints and what are likely to become overriding desires to minimize carbon emissions.
Hire someone with expertise in building science
Very connected to the above two priorities, relative to durability, is to hire someone with expertise in building science. This applies equally to new construction and remodeling. It's complicated--and it's important that your designer and contractor understand what's involved in building (or remodeling) a home in a way that will keep it going strong for hundreds of years.
Summary so far of my top-10 list of green building priorities:
#6. Ensure durability and reuse existing buildings
#7. Protect and restore the site
#8. Use green materials
#9. Create resilient, climate-adapted buildings
#10. Make it easy for homeowners to be green
In addition to this Energy Solutions blog, Alex writes the weekly blog Alex's Cool Product of the Week , which profiles an interesting new green building product each week. You can sign up to receive notices of these blogs by e-mail--enter your e-mail address in the upper right corner of any blog page.
Alex is founder of BuildingGreen, LLC and executive editor of Environmental Building News . To keep up with his latest articles and musings, you can sign up for his Twitter feed